Water filters which remove unwanted minerals, chemicals, bacteria, sediments, etc. have been in wide use for many years. They use a combination of activated charcoal, silver and various types of resin beads as filter means to remove the undesirable elements from the incoming water. Some filters only remove elements related to hardness, like Ca++, Mg++, Cd++, Ba++, Hg++, Na+, Cu+, K+, (positive ions), while others remove Cl-, Br-, I-, (negative ions). Other filters remove both kinds of ions from water and are referred to as de-ionizers. The activated charcoal removes unwanted organic molecules and elements which cause bad taste. Small amounts of silver kills bacteria and water borne viruses. The water conductivity is reduced when the ions and unwanted chemicals are removed from solution after passing through the filtering means. Depending upon the concentration of minerals, chemicals and organisms present in the unfiltered water, the filter life can vary greatly. A filter could last for a month or a year depending on the condition of the water which is being filtered. It would be desirable to find an inexpensive electronic means to measure the filter to determine if it is saturated or used up. After conducting many tests, it was determined that conductivity can be used as a measure of filter effectiveness. When the conductivity of the filtered water is nearly equal to the input water, the resin is saturated and the filter should be replaced. Since water conductivity may vary greatly depending upon local water treatment, geographic location, water temperature, chemical softening and the like, not only will the length of filter life change, but the filter efficiency will produce only a relative drop in conductivity of the filtered water. To cancel out relative filter efficiency, wide ranging input water conductivities, different input water temperatures and other variables, a differential conductivity measurement provides the correct results using a minimum of parts and reduces production costs allowing wide use in several consumer product applications.